1. Field of the Invention
The present invention relates to a control device for a direct current motor, and more precisely, to a speed control device which keeps the number of rotations of a small size direct current motor at a predetermined value and at the same time so controls the prescribed number of rotations of said motor as can be varied.
2. Description of the Prior Art
Heretofore, an arrangement to effect speed control of a motor which has been most widely used is a mechanical governor. However, such mechanical governor has such arrangement as essentially based on its mechanical nature, therefore it is unavoidable to have the temperature of moving parts go up or the parts wearing out, thus making it difficult to maintain accuracy in speed control.
To eliminate the above difficulty, it has been proposed to effect speed control of a motor using an electric control means.
This kind of control device is a general control system to detect the number of rotation of a motor and to compare electrically said detected value with the value of the reference number of rotation, thus controlling the driving voltage of a motor by the output from said comparison.
In this kind of conventional electric control device, the method adopted as a detection system of the number of rotations of a motor is a method to use a tacho generator and a frequency generator being connected directly with a motor, or a method to utilize the counter electromotive motive force of a motor itself, or other various methods have been proposed. On the other hand concerning an electric control circuit provided at a last step of said control process for controlling the driving voltage of a motor, a transistor circuit containing a plural number of control transistors have been generally used in conventionally adopted devices. However, many of the conventional control devices adopt such system that the above mentioned transistor circuit functions in a semi-conductive state and the voltage between a power source and a motor is made to drop by varying the internal resistance of the circuit, causing the terminal voltage of the motor to drop, and the amount of this voltage droppage is made to vary for having the same correspond with and follow the variation in a load. Therefore, the larger a dynamic range is made against the variation in the load, the more ineffective power is wasted at the transistors being connected in series at a stationary state.
Particularly when said device is applied to a speed control system of a portable type small size electrical appliance using a battery or batteries, for example such device as a tape-recorder, or a cine-camera, etc., said loss is great as will be explained below, constituting great shortcomings in regard to the size of a battery or weight of the same.